The ability to detect and measure gases at very low concentrations in an environment has many potential commercial and military uses. For instance, it may be desirable to detect poisonous gases in the air in a battlefield or other environment.
A well-known class of techniques for detecting gases is spectroscopy. Spectroscopy is a scientific technique by which electromagnetic radiation, e.g., light, from a given source is segregated into its wavelength components and the relative light power at each wavelength is analyzed to determine the identity or physical properties of the molecules or atoms of the source of that radiation. Particularly, the wavelengths of radiation that are or are not in the spectrum are indicative of the atoms or molecules that are in the source of the radiation. The term “source” is used broadly herein to encompass not only objects that emit radiation, but also objects that merely reflect, transmit, or absorb radiation emitted by another source.
Within these spectra, one can study emission and/or absorption lines, which are the fingerprints of atoms and molecules. For instance, every atomic element has a particular emission and/or absorption spectrum.
There are several different physical phenomena that can cause any particular molecule or atom to have its particular spectral signature. They include electronic, rotational, and vibrational characteristics. The electronic characteristic refers to the spacing of electron orbits of the atoms. The location and spacing of spectral lines is unique for each atom and, therefore, each atom produces a unique emission or absorption spectrum. The electronic characteristic of a source usually is the dominant characteristic producing the signature spectrum, for instance, in the visible wavelengths.
On the other hand, in the near infrared (IR) range (which is roughly 0.75-3.0 microns), midwave IR range (about 3.0-8.0 microns), and longwave IR range (about 8.0-30 microns], the dominant mechanism responsible for spectral absorption bands are not transitions between electron energy levels, but rather transitions between molecular vibrational energy levels. In the far IR range, sometimes referred to as the Terahertz or THz range (about 30-3000 microns), molecular rotational energy levels are the dominant mechanism.
In the THz regime (far IR), there is an even further physical mechanism that affects the spectral absorption bands. Specifically, solid materials exhibit different spectra based on the absorption spectra of the material's crystalline lattice vibrations (so called phonon spectrum). The principle is the same, but the fundamental mechanism for spectral emissions is lattice vibrations rather than molecular vibrations or rotations.
The overall spectral signature of a gas (or any other object) in any given wavelength band may be the result of any one or more of these phenomena. It does not matter for measurement purposes which physical phenomenon is the cause of any particular spectral line.
Even further, continuous spectra (also called thermal spectra) are emitted by any object that radiates heat, i.e., has a temperature above absolute zero. The light (or other electromagnetic radiation) is spread out into a continuous band with every wavelength having some amount of radiation. Accordingly, the magnitude of radiation at a given wavelength or wavelengths may be used to determine the general composition of an object and/or its temperature or density.
Cavity Ring Down Spectroscopy (CRDS) is one particular spectroscopy technique that can measure the presence and/or concentration of gases in an environment by measuring the absorption spectra of the gas.
Generally, in CRDS, electromagnetic radiation, such light from a laser source, is introduced into a cavity containing the gas to be measured. Mirrors are arranged in the cavity so as to cause the light beam to continuously travel in a loop in the cavity. For instance, a cavity employing two mirrors that causes the light to travel back and forth between the two mirrors continuously can be used. Alternately, three mirrors can be positioned in the cavity to cause the light to continuously travel around the cavity in one direction. A typical CRD cavity could provide an effective path length of several kilometers in a very compact design, e.g., 50 centimeters per side. The light in the cavity will dissipate over a certain period of time as a result of primarily two factors, namely, (1) the absorption of the light by the molecules of the gas in the cavity and (2) the inherent losses of the cavity itself. The inherent losses of the cavity include phenomena such as scatter, less than perfect reflectivity of the mirrors, and absorption by the mirrors and other elements in the cavity that the light strikes. If the inherent losses of the cavity are determined, such as by empirical measurement, their effect on the light dissipation time can be subtracted out of any results, thus leaving the dissipation time caused by absorption of the light by the gas in the cavity.
The rate of absorption of the light by the gas is dictated largely by two factors, namely, (1) the cross-section of the particular gas molecule at the wavelength of the light and (2) the path length of the cavity. The cross-section of the gas molecules at the particular wavelengths at which it will be tested should be empirically or otherwise determined and known ahead of time.
If a particular wavelength corresponds to a spectral peak of the gas, then the decay time will be shorter at that wavelength. The decay time is predictive, not simply of the presence of a particular gas (having a spectral peak at the wavelength been tested), but also its concentration.
Thus, given knowledge of the path length of the cavity, the inherent losses of the cavity, and the pre-known cross-section of the gas molecule at the particular wavelength of the light injected into the cavity, then the length of time necessary for light in the cavity to dissipate can be readily converted into a spectral absorption characteristic for the gas in the cavity. Light at more than one wavelength can be introduced into the cavity sequentially in order to obtain a spectral analysis at several wavelength points.
CRDS systems can be used to detect extremely low concentrations of gases in an environment, as low as a few parts per billion. However, there are environments and circumstances (e.g., battlefield poisonous gas detection) in which it would be desirable to detect the presence of particular gases at much lower concentrations, including a few parts per trillion or a few parts per quadrillion.